Wood structural members such as glue laminated wood beams, parallel strand lumber, laminated veneer lumber, laminated stranded lumber, or solid lumber are used in the construction of buildings and other structures, such as bridges. Wood structural members typically support significant loads in such structures, for example, as beams spanning open areas or as columns supporting other structural members.
The wood structural members in a structure are typically coupled together or to other structural materials by steel plates bolted to exposed faces of the wood structural members. To provide adequate transmission of loads or moments through such couplings, relatively large numbers of bolts (e.g., up to about 40 per member connection) are typically required. Assembling such couplings can be relatively time-consuming and expensive.
The bolt holes in the wood structural members and the steel coupling plates are drilled separately because wood and steel have different boring rates. Drilling them together would typically result in unacceptable damage to the wood structural members. In an attempt to simplify the assembly of wood structural members at the site of construction, bolt holes are typically formed in the steel coupling plates and the wood structural members before they are delivered to the site where the structure is being built. However, prefabricated bolt holes in wood structural members often do not precisely match the prefabricated bolt holes in the corresponding steel coupling plates.
The main reason is that wood structural members frequently undergo dimensional changes between the time they are manufactured and the time they are assembled in a structure. The dimensional changes occur in response to temperature and moisture variations that arise under different weather conditions when the wood structural members are manufactured, transported, and finally assembled. Such dimension changes are typically less than about 0.2 percent and 6 percent of the respective length and width of a wood structural member. However, even such small variations can introduce unacceptable misalignment between prefabricated holes in wood structural members and steel coupling plates.
Another disadvantage of conventional external plate and bolt couplings is that they are relatively inefficient at transmitting loads between structural members. One reason is that relatively large volumes of wood structural members typically must be removed in drilling all of the bolt holes required of a coupling. Another reason is that deformation occurs between the steel coupling plates and the wood structural members, even if large numbers of bolts are used. Slack forms where a coupling has been deformed and increases whenever the deformation is repeated. This can eventually lead to failure of the structural coupling, particularly under seismic or other dynamic load situations.
Another disadvantage of conventional external plate and bolt couplings is that they do not withstand the thermal loads in a fire as well as wood does and therefore lower the overall fire resistance of the structure. A further disadvantages of conventional external plate and bolt couplings are that they are aesthetically displeasing.